The present invention relates to a method for forming an interlayer insulating film and, more particularly, to a method for forming an interlayer insulating film having a low dielectric constant, which is necessary for a highly integrated semiconductor device. Progress in highly integrated semiconductor devices in recent years has resulted in a smaller spacing between wiring layers. Because reduction in the spacing between the wiring layers causes an increase in capacitance between the wiring layers, a need has been created for an interlayer insulating film having a low dielectric constant.
With recent progress in high integration of an LSI device, the wiring has been made finer and multilayered. There has also been an increase in capacitance between the wiring layers. Such an increase in capacitance has caused a great reduction in operating speed. Thus, improvement in this regard has been strongly needed. One method for reducing capacitance between the wiring layers uses an interlayer insulating film having a dielectric constant lower than that of the SiO2 conventionally used for an interlayer insulating film.
Typical interlayer insulating films of low dielectric constants currently under study are (1) SiOF films, and (2) organic insulating films.
(1) SiOF Film
A SiOF film is formed by using a source gas containing F and substituting Sixe2x80x94F bonds for a portion of the Sixe2x80x94O bonds in SiO2. This SiOF film has a relative dielectric constant which is monotonically reduced as concentration of F in the film increases.
For forming such SiOF films, several methods have been reported (see p.82 of the monthly periodical xe2x80x9cSemiconductor Worldxe2x80x9d, February issue of 1996). Most promising among these methods is one using SiH4, O2, Ar and SiF4 as source gases in a high-density plasma enhanced CVD method (HDPCVD method). The relative dielectric constant of an SiOF film formed by this method is in a range of 3.1 to 4.0 (varies depending on F concentration in the film). This value is lower than the relative dielectric constant 4.0 of SiO2, which has conventionally been used for the interlayer insulating film.
(2) Organic Insulating Film of Low Dielectric Constant
Insulating films which have a lower dielectric constant (3.0 or lower) than a SiOF film are organic insulating films. Table 1 shows a few organic insulating films of low dielectric constants, which have been reported, and their respective relative dielectric constants and thermal decomposition temperatures.
However, the SiOF film has the disadvantage that an increase in concentration of F in the film leads to a reduction in moisture absorption resistance. The reduced moisture absorption resistance poses a serious problem, because the transistor characteristic and adhesion to an upper barrier metal layer are adversely affected.
Peeling-off of the organic insulating film of a low dielectric constant easily occurs, because of poor adhesion to a silicon wafer or SiO2 film. Furthermore, the organic insulating film has the disadvantage that its heat resistivity is low. Its thermal decomposition temperature is around 400xc2x0 C. The disadvantage of low heat resistivity poses a problem for annealing a wafer at a high temperature.
It is an object of the present invention to provide a method for forming an interlayer insulating film having good moisture absorption resistance, good heat resistivity and a low dielectric constant, a semiconductor device using the interlayer insulating film, and a semiconductor manufacturing apparatus for forming the interlayer insulating film.
According to the method for forming the interlayer insulating film according to the present invention, as illustrated in FIG. 1C, the film is formed on a substrate by plasma enhanced chemical vapor deposition using a source gas (or a reaction gas) containing a Sixe2x80x94Cxe2x80x94Oxe2x80x94H compound, O2 and B2H6, B (boron), C (carbon) and H2O are contained in the film thus formed. The inventor found that when this film is annealed using an O (oxygen) plasma, C (carbon) and H2O in the film are released from the film and thus many voids are created in the film, as illustrated in FIG. 1D. Thus, a porous SiO2 film containing B (boron) can be formed on the substrate. When the film containing B (boron), C (carbon) and H2O is formed on the substrate, H2O contained in the film may enter into the substrate. This can be prevented in the following manner. That is, as illustrated in FIGS. 1B and 2B, an underlying insulating film is formed on the substrate, and then the film containing B (boron), C (carbon) and H2O is formed.
Moreover, the inventor found that when a film containing a Cxe2x80x94Oxe2x80x94H polymer is formed by plasma enhanced chemical vapor deposition using a source gas containing Sixe2x80x94Cxe2x80x94Oxe2x80x94H compound and H2 (hydrogen) and this film is then annealed using the O (oxygen) plasma, a porous SiO2 film can be formed in the same manner as described above. In this case, the Cxe2x80x94Oxe2x80x94H polymer contained in the film is oxidized by the O (oxygen) plasma, and thus the Cxe2x80x94Oxe2x80x94H polymer is released from the film, and consequently the voids are created in the film.
Furthermore, the inventor found that in forming a film containing the Cxe2x80x94Oxe2x80x94H polymer, if O2 is added to the source, larger voids are created in the film, and the content of SiO2 in the film increases and thus the film is stabilized.
Preferably, the film containing the Cxe2x80x94Oxe2x80x94H polymer has such a thinness that the Cxe2x80x94Oxe2x80x94H polymer is sufficiently oxidized by the O (oxygen) plasma. Therefore, the method of the present invention provides a porous SiO2 film having a desired thickness by alternately repeating the formation of the film containing the Cxe2x80x94Oxe2x80x94H polymer and the oxidization by the O (oxygen) plasma, as illustrated in FIG. 3C.
The porous SiO2 film formed as described above has many voids and thus has a surface area larger than the surface area of the SiO2 film having no void. Because the porous SiO2 film is prone to absorb moisture in the air it is subjected to H (hydrogen) plasma treatment as illustrated in FIGS. 1E, 2E, 3D and 3L. By this treatment, dangling Sixe2x80x94O bonds on the void surfaces are substituted with Sixe2x80x94H bonds. As a result, it is possible to prevent the moisture from being adsorbed on the surface of the voids. Furthermore, a cover insulating film is formed on the porous SiO2 film as illustrated in FIGS. 1H and 2M, whereby it is possible to prevent moisture from being absorbed.
The semiconductor manufacturing apparatus according to the present invention has control means for controlling flow rate control means which, in turn, controls the flow rate of the source gas, and switching means for switching a high-frequency voltage applied to a chamber, as illustrated in FIG. 6.
The control means allows alternately repeating the plasma enhanced chemical vapor deposition and the annealing in one chamber, as illustrated in FIG. 4. That is, during a time period from T1 to T2 in FIG. 4, the source gases (H2, TEOS (Tetra-Ethyl-Ortho-Silicate) and Ar) are introduced into the chamber and the high-frequency voltage is applied to the chamber, whereby plasma enhanced chemical vapor deposition takes place. During the time period from T2 to T3, O2 is applied to the chamber, without the high-frequency voltage, whereby the annealing is performed in an atmosphere of O2.
Furthermore, as illustrated in FIG. 5, this control means allows plasma enhanced chemical vapor deposition and annealing in a plasma atmosphere to be alternately repeated in one chamber. That is, during the time period from T1 to T2 in FIG. 5, the source gases (H2, TEOS, O2 and Ar) are introduced into the chamber and the high-frequency voltage is applied to the chamber, whereby the plasma enhanced chemical vapor deposition takes place. During the time period from T2 to T3, O2 alone is introduced into the chamber which the high-frequency voltage is applied to, whereby the annealing is performed in the plasma atmosphere of O2.